U.S. Pat. No. 6,105,824 discloses an auto shut-off glue gun that includes a timing controller, a heating element, and a trigger. Upon release of the trigger, the timing controller turns off the heating element after a predetermined amount of time.
The timing controller includes an AC to DC converter, a reset switch, a time constant circuit, a reference voltage circuit, an operational amplifier, and a switching element. The reset switch is controlled by the trigger to be switched between an open position and a close position. The time constant circuit is connected in series to the reset switch, and is capable of generating a timing voltage. The reference voltage circuit is capable of generating a reference voltage. The operational amplifier is electrically connected to the time constant circuit and the reference voltage circuit. The witching element is electrically connected to an output end of the operational amplifier. The operational amplifier receives the timing voltage and the reference voltage, and outputs a control signal that is proportional to the difference between the timing voltage and the reference voltage. The witching element receives the control signal and is switched between a flow permitting position and a flow preventing position according to the control signal.
When the trigger is pressed, the reset switch is switched to the open position, and the time constant circuit is charged and outputs the timing voltage that is greater than the reference voltage. The operational amplifier receives the timing voltage and the reference voltage, and outputs the control signal of high value to switch the witching element to the flow permitting position. As a result, the AC to DC converter outputs a signal to turn on the heating element.
When the trigger is released, the reset switch is switched to the close position, and the time constant circuit is discharged and outputs the timing voltage that has a voltage value decreasing with time. When the timing voltage is equal to the reference voltage (i.e., the difference between the timing voltage and the reference voltage is zero), the operational amplifier outputs the control signal of low value to switch the witching element to the flow preventing position. As a result, the AC to DC converter stops outputting the signal to the heating element, and the heating element is turned off.
The complicated controlling mechanism of the auto shut-off glue gun greatly increases manufacturing and maintaining costs of the auto shut-off glue gun.